For a recent project, we developed support for sending automatically-generated HTML emails. Now, most people do this by including a message body with MIME-typetext/html. For extra points, sometimes there’s also a text/plain part alongside the HTML in a multipart/alternative container.

The problem with doing things this way is that you can’t include any images or other resources (such as CSS) as separate parts of the email linked to from the main HTML body-part. For that, you need to use the multipart/related MIME-type. Unfortunately, few commonly-used email clients render multipart/related HTML-plus-resource aggregations well.

We only tried the arrangement where the multipart/related, containing the main HTML page and its associated resources, was a sibling of the text/plain part within the multipart/alternative container. The inverse arrangement, with the multipart/alternative as the main document within the multipart/related part, was something we have yet to experiment with.

Here’s a picture of the structure of our initial attempts:

multipart/alternative
 |
 +-- text/plain
 +-- multipart/related
      |
      +-- text/html
      +-- image/gif
      +-- text/css

This worked reasonably well in Thunderbird and Outlook 2002, but we had consistent reports from our customer that the images and stylesheet would randomly fail to display in Outlook 2003 (SP2). After lots of mucking around trying to get Outlook to either work reliably or fail reliably, we gave up on that line and instead simplified the structure of our emails, putting the CSS styling inline in the HTML HEAD element:

multipart/alternative
 |
 +-- text/plain
 +-- multipart/related
      |
      +-- text/html (with text/css inline in HEAD)
      +-- image/gif

This didn’t work particularly well, either: it seems many email clients ignore styles set in the HEAD element. Finally, we moved to applying CSS styling inline, using a style attribute on each styled element. We were able to use an XSLT transformation to allow us to write clean HTML and apply the CSS style attribute automatically. The final structure of the emails we sent:

multipart/alternative
 |
 +-- text/plain
 +-- multipart/related
      |
      +-- text/html (with text/css copied on to each element!)
      +-- image/gif

This seems to work more-or-less reliably across

• Thunderbird
• Outlook 2002
• Outlook 2003 SP2
• Google Gmail
• MS Hotmail

If I was to do it all again, I’d give serious consideration to the traditional non-multipart text/html solution with images hosted by some public-facing web server. We managed to get our multipart-HTML-emails working acceptably, but only by the skin of our teeth.

References:

• The MIME media type registry
• multipart/alternative: RFC 2045, RFC 2046
• multipart/related: RFC 2387

Comments

On 18 July, 2006 at 12:23 pm, Paul Crowley wrote:

On 18 July, 2006 at 4:25 pm, mikeb wrote:

On 3 July, 2008 at 5:46 pm, tonyg wrote:

On 3 July, 2008 at 5:46 pm, tonyg wrote:

On 17 January, 2009 at 7:48 pm, josh wrote: